Epoxidation of propylene

Abstract: In reacting tertiary butyl hydroperoxide in solution in tertiary butyl alcohol with propylene to produce propylene oxide and tertiary butyl alcohol in a reactor system comprising a first isothermal segment and a second adiabatic segment about 60 to 80 weight percent of the tertiary butyl hydroperoxide is converted in the isothermal segment to provide an intermediate reaction mixture, a recycle stream comprising about 25 to 100 weight percent of the combined weight of the propylene and the tertiary butyl alcohol solution is removed from the intermediate reaction mixture and recycled to the first isothermal segment and the remainder of the intermediate reaction mixture is passed through the adiabatic segment where an additional 20 to 40 weight percent of the tertiary butyl hydroperoxide is converted.

Description: This invention relates to the epoxidation of propylene. More particularly, this invention relates to a process for the reaction of propylene with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol in the presence of a peroxidation catalyst to provide propylene oxide and additional tertiary butyl alcohol. Still more particularly, this invention relates to a process for preparing propylene oxide and tertiary butyl alcohol by reacting tertiary butyl hydroperoxide in solution in tertiary butyl alcohol with propylene in the presence of a soluble molybdenum catalyst, wherein the reaction is conducted in stages involving a reactor system comprising a first isothermal segment and a second adiabatic segment, wherein a feed mixture comprising a recycle stream, propylene, and a tertiary butyl alcohol solution of tertiary butyl hydroperand a molybdenum catalyst are charged to the isothermal segment under reaction conditions sufficient to provide for a conversion of 60 to 80 wt.% of the tertiary butyl hydroperoxide to thereby provide an intermediate reaction mixture, wherein a recycle stream constituting 25 to 100 wt.% of the combined weight of the propylene and tertiary butyl alcohol solution is removed as the recycle stream and wherein the remainder of the intermediate reaction product is charged to the adiabatic segment for the conversion of an additional 20 to 40 wt.% of the tertiary butyl hydroperoxide to reaction products. The reaction of an olefin such as propylene with a hydroperoxide such as tertiary butyl hydroperoxide in solution in a solvent such as tertiary butyl alcohol in the presence of a soluble molybdenum catalyst is disclosed in Kollar U. S. Patent No. 3,351,635. Kollar teaches that in general, from about 0.5 to 100 moles of olefin may be used per mole of hydroperoxide, the preferred molar ratio being within the range of about 2 to about 10 moles of olefin per mole of hydroperoxide. Marquis et al. in U. S. Patent No. 4,891,437 disclose an improvement on the Kollar process in the reaction of propylene with tertiary butyl hydroperoxide wherein the reaction is conducted in a medium composed of 60 wt.% or more of polar components which is formed by utilizing a molar ratio of propylene to tertiary butyl hydroperoxide of about 0.5 to 2 moles of charged propylene per mole of charged hydroperoxide, the reaction being conducted in solution in tertiary butyl alcohol in the presence of a molybdenum catalyst. Variations in the Marquis et al. process and in the preparation of catalysts useful therefore are disclosed in Marquis et al. U. S. Patent No. 4,845,251 and U. S. Patent No. 5,107,067. British Patent No. 1,298,253 discloses a process for the staged reaction of propylene with tertiary butyl hydroperoxide in the presence of tertiary butyl alcohol and a soluble molybdenum catalyst wherein the reaction product from the first stage is fractionated to provide a lighter fraction which is recycled to the beginning of the first stage and a heavier fraction which is charged to the second stage of the epoxidation process. Marquis et al. U. S. Patent No. 4,992,566 and Marquis et al. U. S. Patent No. 5,093,506 disclose single stage processes for the reaction of propylene with tertiary butyl hydroperoxide wherein the reaction product is fractionated to provide a propylene stream for recycle. Russell U. S. Patent No. 3,418,340 discloses a process for the production of propylene oxide by the reaction of propylene with tertiary butyl hydroperoxide involving a recycle of the propylene wherein the propylene is further fractionated to remove oxygen prior to recycle. Stein et al. U. S. Patent No. 3,849,451 disclose a process wherein propylene is reacted with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol in a single stage compartmented reactor operated under conditions such that propylene and propylene oxide are vaporized during the course of the reaction and wherein the vapors are recovered and distilled to provide a propylene fraction for recycle. D'Aubigne et al. U. S. Patent No. 4,002,687 disclose a process for the hydroperoxidation of a hydrocarbon such as isopentane or isobutane with oxygen using a plurality of stages with water washing between stages. Background Information Tertiary butyl hydroperoxide is a comparatively stable hydroperoxide. Nevertheless, it is a peroxide and subject to explosive autoxidation under improper conditions. The reaction of tertiary butyl hydroperoxide with propylene is a highly exothermic reaction, liberating about 60,000 calories of heat per gram mole of epoxide formed by the primary reaction. It is therefore of prime importance to maintain positive and accurate control of the reaction temperature during the epoxidation process in order to prevent a run away reaction. It is known to achieve reaction control by conducting the reaction in a heat exchanger in indirect countercurrent contact with the heat exchange medium in order to remove excess heat of reaction as rapidly and efficiently as possible. It is also known to moderate reaction temperature by conducting the reaction in the presence of a solvent. Classically, both techniques have been utilized in the past in the preparation of propylene oxide from tertiary butyl hydroperoxide by conducting the reaction using a comparatively large quantity of solvent and also a large molar excess of propylene to tertiary butyl hydroperoxide, such as a molar ratio of more than 3 moles (e.g., 5 to 10 moles) of propylene per mole of tertiary butyl hydroperoxide. It is also known to conduct the reaction in a reactor provided with heat exchange control means, such as a jacket for the reactor or through the provision of internal immersed cooling coils or both. Another problem that is encountered in the reaction of propylene with tertiary butyl hydroperoxide relates to selectivity. Although the primary reaction of the tertiary butyl hydroperoxide with propylene will result in the formation of propylene oxide and tertiary butyl alcohol, other oxygenated by-products can be and are formed in various amounts, including aldehydes, organic acids, esters, ketones, etc. Moreover, the propylene oxide that is formed can react with oxygenated by-products present in the reaction medium to further decrease the yield of propylene oxide. Another problem as pointed out in Marquis et al. U. S. Patent No. 4,891,437, is the tendency of propylene to react with itself under epoxidation conditions to form an addition product containing 6 or more carbon atoms. These hydrocarbon by-products are particularly pernicious in that they tend to codistill with and to be present in the final propylene oxide product where they significantly and adversely affect its quality. This problem can be minimized by use of a comparatively low mole ratio of propylene to tertiary butyl hydroperoxide, such as a molar ratio of about 1 to about 2 moles of propylene per mole of tertiary butyl hydroperoxide. In accordance with the present invention, the foregoing and other problems are ameliorated through the provision of a process wherein propylene is reacted with tertiary butyl hydroperoxide in a staged process in a reactor system comprising a first isothermal segment and a second adiabatic segment, wherein the isothermal segment is charged with propylene, with a tertiary butyl alcohol solution of tertiary butyl hydroperoxide and molybdenum catalyst and a recycle stream, wherein the reaction conditions in the isothermal segment are established so as to convert 60 to 80 wt.% of the tertiary butyl hydroperoxide to an intermediate reaction mixture, wherein a portion of the intermediate reaction mixture, constituting 25 to 100 wt.% of the combined weight of the initial propylene and tertiary butyl alcohol solution, is removed from the intermediate reaction mixture as the recycle stream, wherein the remainder of the intermediate reaction stream is charged to the adiabatic segment of the reactor system in order to convert an additional 20 to 40 wt.% of the tertiary butyl hydroperoxide to propylene oxide and other products. Propylene oxide and tertiary butyl alcohol are the principal reaction products and can be recovered by distillation. Any excess propylene can also be recovered for recycle as part of the initial propylene charge stream. In accordance with the preferred embodiment of the present invention, the process is carried out continuously in a reaction zone comprising a first isothermal segment and a second adiabatic segment, and wherein: a) a tertiary butyl alcohol solution is prepared containing 35 to 60 wt.% of tertiary butyl hydroperoxide, correspondingly 65 to 40 wt.% of tertiary butyl alcohol, and 100 to 1,000 ppm of a soluble molybdenum catalyst, b) there is continuously charged to the isothermal segment a feed mixture comprising propylene, the tertiary butyl alcohol solution, and also a recycle stream comprising 25 to 100 wt.% of the combined weight of the propylene and the tertiary butyl alcohol solution, the propylene being charged in an amount sufficient to provide an initial charge ratio of 1 to 3 moles of propylene per mole of tertiary butyl hydroperoxide, c) the feed mixtures are continuously passed through the isothermal segment under epoxidation reaction conditions of time and temperature sufficient to convert 60 to 80% of the tertiary butyl hydroperoxide to reaction products including propylene oxide, tertiary butyl alcohol and oxygenated by-products and to provide an isothermal segment reaction product, d) the said recycle is continuously removed stream from the isothermal segment reaction product to provide an intermediate reaction mixture comprising the remainder of the isothermal segment reaction product, e) the recycle stream is continuously returned to the isothermal segment, f) the remainder of the intermediate reaction mixture is continually charged to the adiabatic segment and is flowed therethrough under adiabatic epoxidation reaction conditions of temperature and time sufficient to convert an additional 20 to 40% of the charged tertiary butyl hydroperoxide to reaction products including propylene oxide, tertiary butyl alcohol and oxygenated by-products, g) propylene oxide and tertiary butyl alcohol are continually recovered from the final product. The epoxidation reaction conditions utilized in the isothermal epoxidation segment may suitably include a temperature of 50° to 180°C. and a reaction time of 0.3 to 3 hours. More preferably, the reaction will be conducted at a pressure of 2.8 to 5.6 MPa (400 to 800 psig), a reaction temperature of 80° to 140°C. and a reaction time of 0.5 to 2 hours. Reaction conditions are preferably adjusted in the isothermal segment to provide for a 65 to 75% conversion of the tertiary butyl hydroperoxide and for the recycle of a recycle stream comprising 25 to 100 wt.% of the combined weight of the propylene charge stream and the tertiary butyl alcohol solution. A problem involved in the recycle of the total reactor effluent from the isothermal segment arises in that the recycle stream is at a temperature of 100° to 140°C and contains tertiary butyl hydroperoxide and possibly other hydroperoxide reaction products which may be less thermally stable than the tertiary butyl hydroperoxide. Therefore, when a pump is utilized to provide the motive force for recycling the recycle stream, care must be taken to avoid a potential source of ignition, such as a spark generated by the equipment. Failure of a mechanical pump can also lead to a runaway reaction, especially if olefin impurities such as butylenes and/or pentalenes are present. A mechanical pump may provide an ignition source in the event of the failure of a bearing, a rotor or some other part which permits the creation of a local hot spot. Also, it is not uncommon for a mechanical pump to experience deterioration of the seals which can lead to the release of environmentally noxious or hazardous materials. In accordance with a modified form the present invention, these and other problems may be ameliorated through the provision of a jet pump or eductor in order to provide the motive force for transporting the recycle stream from the effluent end of the adiabatic segment to the charge end. The invention is described with reference to the accompanying drawings. In the drawings, Fig. 1 is a schematic flow sheet illustrating the general reaction sequence of the present invention. Fig. 2 is a schematic flow sheet illustrating the embodiment of the present invention wherein an eductor is utilized to transport the recycle stream. Fig. 3 is a plan view, partially in section, of an eductor of the type that may be used in the process of the present invention. Turning now to the drawings, and in particular to Fig. 1, there is shown a schematic flow sheet illustrating a preferred method of practicing the process of the present invention. In the drawing, conventional parts, such as valves, pumps, temperature sensors, pressure sensors, heaters, coolers and flow control regulation apparatus have been omitted. In accordance with this embodiment of the present invention, there is provided a reactor system comprising an isothermal segment designated generally by the numeral 10 and an adiabatic segment designated generally by the numeral 20. The isothermal segment 10 will comprise a plurality of reactors 12-14, each provided with appropriate indirect heat exchange control means. In order to initiate the process, propylene is charged to the initial reactor 12 by way of a charge line 21 together with a tertiary butyl alcohol solution of tertiary butyl hydroperoxide containing a molybdenum catalyst which is charged by a line 22 and with a recycle stream 24. The resultant reaction mixture is passed through the series of reactors 12-14 in the isothermal segment under conditions of temperature, pressure and time sufficient to convert from 60 to 80 wt.% of the charged tertiary butyl hydroperoxide. The reaction conditions to be utilized in the isothermal segment may include a pressure sufficient to maintain the reactants in liquid phase, such as a pressure of 2.8 to 5.6 MPa (400 to 800 psig), a reaction temperature of 50° to 180°C., such as a temperature of 100° to 130°C., and a reaction time of 0.3 to 3 hours, and more preferably from 0.5 to 2 hours. The reactor effluent from the last of the isothermal reactors 14 is discharged by a line 30 which is provided with a branch line 24 from which the recycle stream is removed and with a second line 32 leading to an appropriate heat exchange means 34 for adjusting the temperature, if necessary, of the resultant remainder of the intermediate reaction mixture after which it is charged by a line 36 to an adiabatic reactor 38 in the adiabatic reaction segment 20 to convert an additional 15 to 20 wt.% of the tertiary butyl hydroperoxide therein. The products of the reaction are discharged in the adiabatic reaction segment 38 by a line 39 leading to a recovery zone 40 wherein the products of the reaction may be separated by conventional distillation means, such as those described in Marquis et al. U. S. Patent No. 4,992,556 or Marquis et al. U. S. Patent No. 5,093,506. For example, the recovery zone 40 may comprise a plurality of serially connected distillation columns wherein the products of the reaction are separated into a propylene fraction 41 that can be recycled by any suitable means (not shown) as part of the propylene charge 21, a propylene oxide fraction 42, a tertiary butyl alcohol fraction 43 comprising charged tertiary butyl alcohol and tertiary butyl alcohol formed during the process, and a heavier fraction 44 comprising reaction byproducts and catalyst residue. By way of example, 450 kg (1000 lb) per hour of propylene may be charged to the isothermal segment by the line 21 together with 1100 kg (2430 lb) per hour of a solution of tertiary butyl hydroperoxide in tertiary butyl alcohol which is charged by the line 22. Suitably, the solution will contain 55 wt.% of tertiary butyl hydroperoxide. A recycle stream charged by the line 24 may comprise, for example, about 166 kg (370 lb) per hour of propylene, about 162 kg (3601b) per hour of propylene oxide, about 504kg (11201b) per hour of tertiary butyl alcohol and about 108kg (2401b) per hour of tertiary butyl hydroperoxide. The feed mixture is passed through the isothermal reactor segment reactors 12-14 under reaction conditions including a temperature of 80° to 140°C., a pressure of 2.8 to 5.6 MPa (400 to 800 psig) and a total reaction time within the isothermal segment of 0.3 to about 2 hours. The effuent of about 2470kg (5490 lb) per hour discharged by the line 30 is separated into 927kg (20601b) per hour of recycle and 1543kg (3430 lb) per hour of feed to the adiabatic reaction segment 38 by way of the lines 32 and 36. Turning now to Fig. 2, there is shown a preferred embodiment of the present invention wherein an eductor is utilized to transport the recycle stream from the isothermal reactor effluent to the beginning of the process. Turning now to Fig. 2, there is shown a reaction system comprising an isothermal reactor segment 50 and an adiabatic reactor segment 60. There is also provided a feedstock preparation tank designated generally by the number 70 to which tertiary butyl hydroperoxide is charged by a line 72 and where it is maintained at a temperature of not more than 60°C. A catalyst solution, such as a molybdenum/ethylene glycol complex in solution in ethylene glycol is also charged to the preparation tank 70 by a line 74, the reactants being suitably charged in proportions such that the feed in the tank 70 contains about 35 to about 60 wt.% of tertiary butyl hydroperoxide, correspondingly about 65 to about 40 wt.% of tertiary butyl alcohol and 100 to 1,000 ppm of the molybdenum catalyst solution. The feed mixture may be discharged from the feed tank 70 by a discharge line 77 leading to a pump 78 of any conventional construction for charging the tertiary butyl alcohol solution by a feed line 79 to a charge line 80 leading to the isothermal segment 50. Propylene is charged to the process by a propylene charge line 82 passing through an eductor 84 of any conventional construction wherein the eductor orifice size, differential pressure and flow rate of the propylene are adjusted so as to educt a desired amount of a recycle stream into the eductor by way of a line 86. Suitably, the quantity of material educted from the line 86 will constitute from 25 to 100 wt.% of the combined weight of the propylene feed in the line 82 and the tertiary butyl alcohol feed in the line 79 and, still more preferably, from 50 to 80 wt.%. Line 80 may suitably lead to a preheater 90 of any conventional construction wherein the feed mixture is brought to a desired reaction temperature by a heat exchange medium passed through the heat exchange preheater 90 by way of a line 92. The heated material is then passed by a line 94 to a first isothermal reactor 96 which may, for example, be a jacketed reactor wherein the reactants are immersed in a heat exchange fluid such as water which is passed through the reactor 96 by way of a heat exchange charge line 98 and which discharges from the reactor by a heat exchange discharge line 99. By adjusting the temperature or flow of the heat exchange medium 92, the flow rates of line 82 and the line 79, appropriate reaction conditions can be established in the isothermal segment including a pressure sufficient to maintain the reactants in liquid phase, such as a pressure of 2.8-5.6 M (400 to about 800 psig), a reaction temperature of 50° to 180°C., and more preferably from 80° to 140°C., and a reaction time of 0.3 to 3 hours, such as a reaction time of 0.5 to 2 hours. The isothermal segment will comprise a plurality of the heat exchange reactors 96, the last reactor in the series being a reactor 96' through which a heat exchange medium is passed by way of heat exchange charge line 98' and discharged by way of heat exchange discharge line 99', the reactor effluent from the first of the isothermal segment reactors 96 is discharged by a line 97 leading to the next succeeding reactor and hence in sequence until the final charge mixture is charged by a line 97' to the heat exchange reactor 96'. The final reactor effluent is discharged from the reactor 96' by a line 100 provided with a branch line 86 from which the desired quantity of reactor effluent is educted for recycle through the eductor 84. Suitably, the amount of educted material will constitute 25 to 100 wt.% of the combined weight of the propylene and tertiary butyl alcohol solution and, more preferably, from 50 to 80 wt.%. The remainder of the intermediate reaction mixture discharged by the line 100 will be charged by way of a line 102 leading to a heat exchanger 104 provided with an indirect heat exchange means such as a flow line 106 for adjusting the temperature of the intermediate reaction mixture, after which it is charged by way of a line 108 to the adiabatic segment reactor 110 where an additional 20 to 40% of the tertiary butyl hydroperoxide is converted. Thereafter, the reactor effluent is discharged from the reactor segment 110 by a line 112 leading to a distillation zone (not shown) wherein the reaction effluent may be fractionated into suitable fractions such as a propylene fraction which can be recycled by suitable means (not shown) as part of the propylene charge 82, a propylene oxide product fraction, a tertiary butyl alcohol fraction comprising charged tertiary butyl alcohol and tertiary butyl alcohol formed during the process, and a heavier fraction comprising the reaction by-products and catalyst residue. Turning now to Fig. 3, there is shown the design of an eductor of the type that can be used in the process of the present invention. It will be understood that the eductor shown in Fig. 3 is given by way of example and that eductors of a different design are commercially available and can be used in the process of the present invention such as a Fisher spike nozzle model 12TC or a Fisher standard model 12TC. As shown in Fig. 3, the eductor or jet pump comprises a housing 201 defining a chamber 202 having an inlet opening 204 through which liquid may be educted into the chamber 202. A flange or any other suitable means 206 is provided for securing an inlet pipe to the opening 204. A charge line 210 is provided entering from the rear of the housing 200 and terminating inside the chamber 202 in an eduction nozzle segment 212. Suitable means such as a tap 214 is provided for securing the housing 200 to a support mechanism through which the charge line 210 may be passed. The eductor 200 is also provided with a discharge barrel 230 having an outwardly flaring discharge channel 232 into which the charged liquid from the charge line 210 and the liquid from the chamber 202 are mixed and discharged. Suitable fastening means such as a flange 234 may be provided for securing the barrel to a discharge pipe (not shown). By adjusting the discharge angle, the sides of the throat 240 at the entrance of the barrel, the shape of the nozzle 212 and the tolerance between the two, it is possible to provide for a motive force sufficient to move a desired quantity of liquid from a suitable source (not shown) through the inlet line 204 to the chamber 202 and thence in admixture with charge liquid from charge line 210 into the barrel 232 for discharge from the system.

Claims: A process for preparing propylene oxide and tertiary butyl alcohol by reacting propylene with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol in the presence of a soluble peroxidation catalyst, which comprises: conducting said process in a reactor system comprising a first isothermal segment and a second adiabatic segment, charging to said isothermal segment a feed mixture comprising a recycle stream, propylene and a tertiary butyl alcohol solution of tertiary butyl hydroperoxide and peroxidation catalyst, and converting 60 to 80 weight percent of the tertiary butyl hydroperoxide in the isothermal segment to provide an intermediate reaction mixture, removing a recycle stream, constituting 25 to 100 weight percent of the initial propylene and tertiary butyl alcohol solution, from the intermediate reaction mixture and recycling it to the first isothermal segment as said recycle stream, and passing the remainder of the intermediate reaction mixture through the adiabatic segment and converting an additional 20 to 40 weight percent of the tertiary butyl hydroperoxide therein. A process as claimed in Claim 1, wherein the feed mixture comprises from 35 to 60 weight percent of tertiary butyl hydroperoxide, from 65 to 40 weight percent of tertiary butyl alcohol, and from 100 to 1000 ppm of soluble molybdenum catalyst, optionally as a complex with ethylene glycol. A process as claimed in Claim 2, wherein the process is conducted on a continuous flow basis, which comprises: a. preparing a tertiary butyl alcohol solution containing 35 to 60 weight percent of tertiary butyl hydroperoxide, correspondingly 65 to 40 weight percent of tertiary butyl alcohol and 100 to 1,000 ppm of a soluble molybdenum catalyst, b. continuously charging to said isothermal segment a feed mixture comprising propylene, said tertiary butyl alcohol solution and a recycle stream comprising 25 to 100 weight percent of the combined weight of said propylene and said tertiary butyl alcohol solution, said propylene being charged in an amount sufficient to provide an initial charge ratio of 1 to 3 moles of propylene per mole of tertiary butyl hydroperoxide, c. continuously passing said feed mixture through said isothermal segment under epoxidation reaction conditions of time and temperature sufficient to convert 60 to 80 weight percent of the tertiary butyl hydroperoxide to reaction products including propylene oxide and tertiary butyl alcohol, and to provide an isothermal segment reaction product, d. continuously removing said recycle stream from said isothermal segment reaction product to provide an intermediate reaction mixture comprising the remainder of said isothermal segment reaction product, e. continuously returning said recycle stream to said isothermal segment, f. continuously charging said intermediate reaction mixture to said adiabatic segment and flowing said intermediate reaction mixture therethrough under adiabatic epoxidation reaction conditions of temperature and time sufficient to convert an additional 20 to 40 weight percent of the charged tertiary butyl hydroperoxide to reaction products, principally propylene oxide and tertiary butyl alcohol to provide a final reaction product, and g. continuously recovering propylene oxide and tertiary butyl alcohol from said final reaction product, said epoxidation reaction conditions in said isothermal segment and said adiabatic segment of said epoxidation reaction zone including a temperature of 50° to 180°C, and a reaction time of 0.3 to 3 hours. A process as claimed in any one of claims 1 to 3, wherein the reaction conditions are adjusted in the isothermal segment to provide for a 65 to 75 percent conversion of the tertiary butyl hydroperoxide, and wherein the recycle stream comprises 50 to 80 weight percent of the combined weight of the charged propylene and tertiary butyl alcohol solution. A process as claimed in claim 4, wherein the epoxidation reaction conditions include a temperature of 80° to 140°C, a pressure of 2.8 to 5.6 MPa (400 to 800 psig) and a reaction time of 0.5 to 2 hours in the isothermal segment.

Inventors: MUELLER MARK ALLAN, PRESTON KYLE LEE, TAYLOR MARK ELLIOTT, WU CHUNG NAN TSAI, MUELLER, MARK ALLAN, PRESTON, KYLE LEE, TAYLOR, MARK ELLIOTT, WU, CHUNG NAN TSAI
Applicants: HUNTSMAN SPEC CHEM CORP, HUNTSMAN SPECIALTY CHEMICALS CORPORATION
Application year: 1994
Main classification: C07D301
IPC codes: C07B61, B01J23, B01J31, C07D303, C07D301